1. Field of the Invention
The present invention relates to a receiver coil for receiving and collecting nuclear magnetic resonance signal in a nuclear magnetic resonance signal.
2. Description of the Background Art
As a receiver coil for a nuclear magnetic resonance imaging apparatus, various types of the receiver coil have been developed for different imaging method and different imaging object shapes. In particular, there are many examples of a receiver coil developed for imaging a spine at a neck portion, a chest portion, or a stomach portion of a patient.
Examples of such a conventional receiver coil are shown in FIGS. 1(A) to 6.
An example of a receiver coil shown in FIGS. 1(A) and 1(B) is called a crossing elliptic coil, and this receiver coil 1 comprises a pair of elliptic coil elements 1a and 1b which are wound around a cylindrical core 1c and are arranged such that they are crossing with each other and electrically connected at cross sections P.sub.1 and P.sub.1 ', where vertical positions of the cross sections P.sub.1 and P.sub.1 ' coincide with that of a center of the cylindrical core 1c, as shown in FIG. 1(B). In this configuration, a sensitivity central axis L.sub.S1 of a region of uniform signal reception sensitivity and a central axis L.sub.G1 of the cylindrical core 1c coincide with each other.
As shown in FIGS. 2(A) and 2(B), the receiver coil 1 of the above example can be modified into a receiver coil 1' which comprises a pair of elliptic coil elements 1a' and 1b' which are wound around a cylindrical core 1c' and are arranged such that they are crossing with each other and electrically connected at cross sections P.sub.2 and P.sub.2 ', where vertical positions of the cross sections P.sub.2 and P.sub.2 ' coincide with that of a center of the cylindrical core 1c', as shown in FIG. 2(B), and where an angle formed between the elliptic coil elements 1a' and 1b' are made narrower than that between the coil elements 1a and 1b in the previous example. In this configuration, a sensitivity central axis L.sub.S1 ' of a region of uniform signal reception sensitivity and a central axis L.sub.G1 ' of the cylindrical core 1c also coincide with each other, as in the previous example. With this configuration, a closer fitting to a patient's body surface becomes possible.
Another example of a conventional receiver coil is shown in FIG. 3, which is called a saddle coil, and this receiver coil 2 comprises a saddle shaped coil element 2a wound around a cylindrical core 2b. In this configuration, a sensitivity central axis L.sub.S2 of a region of uniform signal reception sensitivity and a central axis L.sub.G2 of the cylindrical core 2b also coincide with each other, as in the previous examples.
Another example of a conventional receiver coil is shown in FIG. 4, which is another example of a saddle coil, and this receiver coil 3 comprises a pair of semi-saddle shaped coil elements 3a wound around a cylindrical core 3b. In this configuration, a sensitivity central axis L.sub.S3 of a region of uniform signal reception sensitivity and a central axis L.sub.G3 of the cylindrical core 3b also coincide with each other, as in the previous examples.
Another example of a conventional receiver coil is shown in FIG. 5, which is called a Helmholtz coil, and this receiver coil 4 comprises a pair of circular coil elements 4a and 4b arranged with a prescribed separation formed therebetween. In this configuration, a sensitivity central axis L.sub.S4 of a region of uniform signal reception sensitivity and a central axis L.sub.G4 passing between the circular coil elements 4a and 4b also coincide with each other.
Another example of a conventional receiver coil is shown in FIG. 6, which is called a modified Helmholtz coil, and this receiver coil 5 comprises a pair of warped circular coil elements 5a and 5b arranged with a prescribed separation formed therebetween. In this configuration, a sensitivity central axis L.sub.S5 of a region of uniform signal reception sensitivity and a central axis L.sub.G5 passing between the circular coil elements 5a and 5b coincide with each other, as in the last example.
In all of these examples, the coil elements are supported by a core member formed by a non-magnetic resinous supporting material which maintains the geometrical configuration of the coil elements.
Now, in these conventional receiver coils, the region of uniform signal reception sensitivity is a space formed in an opening section of the coil elements. The condition for a satisfactory signal reception is to make this region as large as a region of interest to be imaged, and to place this region of interest at a center of the region of uniform signal reception sensitivity.
As an example, the operation of the receiver coil 1 of FIGS. 12(A) and 12(B) will now be described with references to FIGS. 17 and 18.
This receiver coil 1 is one for imaging head and neck portions, and has a diameter of opening section approximately equal to 260 mm, such that it can be used for both an imaging of a head portion and an imaging of a neck portion.
As shown in FIG. 17, in a case of imaging the head portion H, the head portion H is inserted inside the opening section of the receiver coil 1 completely. In this case, the region of uniform signal reception sensitivity coincide with a size of the head portion H, and a center of the head portion H is located on the sensitivity central axis L.sub.S1 of the receiver coil 1, so that the condition for the satisfactory signal reception is fulfilled, i.e., the nuclear magnetic signals can be received with a high S/N ratio.
However, as shown in FIG. 18, in a case of imaging the neck portion N, when the neck portion N is inserted inside the opening section of the receiver coil 1 completely, the central axis Na of the neck portion N is located below the sensitivity central axis L.sub.S1 of the receiver coil 1, so that the condition for the satisfactory signal reception cannot be fulfilled completely, i.e., the nuclear magnetic signals can be received only with a low S/N ratio.
Thus, in a conventional receiver coil, because the coil elements are wound such that the sensitivity central axis and the central axis of the opening section coincide with each other, the condition for a satisfactory signal reception cannot be fulfilled completely for the imaging of a region of interest that cannot be placed symmetrically with respect to the central axis of the receiver coil such as a neck portion. In particular, the neck portion usually ends up being placed off the sensitivity central axis of a conventional receiver coil, so that the quality of the imaging of the neck portion has usually been poorer.